Does Eclipse stack with roar?

You cannot put another damage buff ability onto a frame that has a damage buff, unless you replace the ability. So you cannot put eclipse on rhino unless you replace roar. You can however replace one of rhino's ability with Oberon's Smite/Ember's Fireball/Volt's Shock and use their augment to make it into a damage buff, as that's allowed.

If you get eclipse/roar from another warframe, while you have the other. They do both work.